DESCRIPTION: (Investigator's Abstract) An increase in total body fat as well as a central fat distribution confer risk for the development of diabetes and atherosclerosis. A major regulator of fat deposition is the rate of uptake of circulating triglycerides which is controlled by the activity of the enzyme lipoprotein lipase (LPL). LPL activity (LPLA) undergoes rapid changes with nutritional status and varies with adipose depot and gender. The cellular bases for nutritionally-induced and depot variations in LPLA in human adipose tissue are unknown and will be investigated. Mechanisms underlying in vitro regulation of LPL by specific hormones will be compared with in vitro regulation in response to nutritional influences. LPL mRNA levels and LPL mass in heparin eluates will be measured in abdominal and gluteal adipose tissue of men and women undergoing nutritional manipulations -semistarvation and overfeeding-known to markedly affect LPLA. Because insulin and glucocorticoids are thought to be the major hormonal mediators of nutrition-induced changes in LPLA, their mechanism of action will be studied in vitro. This will be done using organ culture of human adipose tissue fragments, which allows maintenance of hormonally-responsive adipose tissue fragments for periods of up to 2 weeks. Preliminary data show that insulin alone and acting synergistically with glucocorticoids increase LPLA in human adipose tissue maintained in organ culture. The effect of glucocorticoid is accounted for by a decrease in the rate of degradation of newly-synthesized LPL. The mechanism(s) of action of these hormones will be further investigated by studying the dose-response relationships and time course of varying concentrations of insulin, in the absence or presence of varying concentrations of glucocorticoids, on levels of LPL mRNA, rates of LPL synthesis and degradation and LPL mass. In addition, the influence of hormones on degradation of exogenously added 125I-LPL will be studied. The role of a lysosomal pathway in LPL degradation will be evaluated with specific inhibitors. Since LPL acts extracellularly and its secretion may be regulated, the influence of insulin on the tissue distribution of newly-synthesized LPL, and the rate of LPL secretion will be studied. The hypothesis that an abdominal fat distribution results, in part, from increased responsiveness or sensitivity to effects of insulin and glucocorticoids on LPL will be examined by comparing responses in adipose tissue subcutaneous abdominal, gluteal and intraabdominal adipose tissues of men and women. Potential alterations in in vitro LPL regulation in adipose tissue from obese subjects will also be studied. These studies will allow comparison of in vivo and in vitro LPL regulation in humans and will provide insight into the molecular mechanisms controlling site-specific fat accumulation.